Eddy currents provide a measurable indicator of flaws in the surface and sub-surface of conductive materials. They are generally confined to the surface and near surface regions of the material. They are affected by changes in the resistivity of the conductive material. Flaws in the material, such as microscopic hair line cracks or pits, affect the localized resistivity of the material. Flaws in a material cause localized variations in the eddy currents in the material. Accordingly, a conductive material can be inspected for flaws by inducing and measuring eddy currents in the material.
Eddy current probes detect material flaws by sensing variations in eddy currents. These probes have coils with high frequency currents that project a fluctuating magnetic field into the conductive material being measured. This imposed magnetic field induces eddy currents in the material. The strength of the eddy currents depends on the local resistivity of the material which resistivity is affected by the presence of material flaws and cracks. The eddy currents create a magnetic field that varies in intensity with the strength of the eddy current and, hence, the presence of material flaws.
The magnetic field created by the eddy currents extends above the material surface up to the probe. The magnetic field from the eddy current induces its own voltage in the probe coil. The eddy magnetic field opposes the coil field. These coupled magnetic fields measurably influence the net current and inductance of the probe coils. These variations in the coil currents vary in response to material flaws are measured to detect these flaws.
For the probe coil current to reliably indicate variations in eddy currents, other parameters that affect the coil current must be held constant. One such parameter is the distance between the face of the probe and the surface of the material. The degree of coupling between the magnetic fields from the coil current and the eddy current depends on the gap between the probe and the material with the eddy currents. The gap between the probe and material is known in the art as being the "lift-off" of the probe.
Changes in the lift-off gap alter the amount of magnetic coupling and the coil current in the eddy current probe. An eddy current probe is influenced by current alterations due to gap changes as it is influenced by current variations due to material flaws. Since it is desired to detect only eddy current variations due to material flaws, current variations due to changes in the lift-off gap must be segregated from variations clue to material flaws.
It is difficult to maintain a constant gap distance between the probe and the material being tested. It is particularly difficult to maintain a constant gap when a large surface, such as a retaining ring for a power generator, is being tested. Retaining rings are large and typically have radii in the range of 13 to 36 inches.
Retaining rings are not perfect cylinders, because they are large and their surfaces have been reworked from earlier maintenance. During the Life of a retaining ring, it may be removed several times from the generator rotor shaft for reworking. Each time a retaining ring is reinserted onto the generator shaft by shrink fitting, the ring deforms slightly to conform to the slotted surface of the rotor. Accordingly, the surface of the retaining ring becomes more irregular each time the ring is reworked.
The surface of a retaining ring is immense compared to the small material flaws that an eddy current probe detects. Large retaining rings do not have surfaces that are uniform at the small order of magnitude (microcracks) at which the eddy currents are being measured. The irregularities in the shape and surface of the retaining ring make it difficult to hold the probe a constant distance above the surface of tile ring.
Eddy current probes are usually fixed with respect to a known reference other than the retaining ring. A true and known reference is necessary to precisely position the probe with respect to the retaining ring. The retaining ring usually bears a stamp on its end surface marking the zero degree position of the ring. The position the eddy current probe is referenced from this zero reference stamp. A reference for the probe is established with a conventional reference frame. This reference frame is attached to the retaining ring and is centered on the axis of the ring as shown in FIG. 1. The eddy current probe is affixed to the reference frame and positioned near the surface of the retaining ring. The reference frame is motorized so that the eddy current probe can be drawn across the surface of the retaining ring. Generally, the probe is moved axially along the length of the retaining ring in a straight scan line.
As the probe completely traverses each scan line across the retaining ring surface, the probe is circumferentially indexed to the next scan line around the reference frame. The probe is then drawn in reverse along this next scan line. This scanning and indexing sequence is repeated until the probe completely scans the entire circumference of the retaining ring. In this way, the probe covers the entire surface of the retaining ring. The probe must cover the entire ring to ensure that all material flaws are detected. To do this, the probe travels along straight scan lines parallel to the axis of the retaining ring. If the probe wanders off a scan line, then portions of the material surface will be missed by the probe and flaws in the material may escape detection. Moreover, it is difficult to accurately specify the location of flaws when the probe drifts off the intended scan line.
Prior to the present invention, eddy current probes generally scanned a retaining ring with a single probe one scan line at a time. This prior scanning method was inordinately slow because the one probe had to cover the entire retaining ring. The one line at a time scanning method was previously believed to be the only method suited for scanning retaining rings and other applications where the probe was rotated about a stationary surface.
The use of multiple probes in a single carriage was known for eddy current scanning where the surface rotated relative to the probe. For example, U.S. Pat. No. 4,258,319 discloses a carriage with a plurality of eddy current probes that scan a rotating shaft. However, the use of multiple probes was previously limited to scanning outside rotating surfaces because of complex and bulky carriages that carried the probe. Accordingly, a long-felt need existed for faster methods of scanning the interior surfaces of retaining rings with eddy current probes.
In the present invention, a carriage holds a plurality of eddy current probes held a fixed distance above the retaining ring surface. The carriage has self-lubricating feet that slide across the surface of a retaining ring. The carriage moves in a straight line along the surface of the retaining ring. As the carriage moves, the eddy current probes trace a plurality of parallel scan lines across the surface of the retaining ring. The probes in the carriage are usually separated by a distance greater than the desired distance between scan lines. This separation of probe is necessary to prevent cross-talk between the electromagnetic fields of the probes, and due to the size of the probes and probe mounting of the carriage. A novel scanning method was required to achieve the desired narrow spacing of scan lines with a multiple probe carriage.
In operation, once the probe carriage traverses the ring surface in one direction, the carriage is shifted incrementally sideways by a distance equal to the width of the row of probes held in the carriage. The carriage then traverses the surface in an opposite direction. By shifting sideways the carriage and scanning the ring surface back and forth, the carriage and probes cover the entire surface of the retaining ring. However, the distance between scan lines after one rotation of the carriage around the retaining ring is the distance separating the probes on the carriage to achieve finer spacing of scan lines, the carriage must rotate around the retaining ring more than once.
When the probe carriage begins the second rotation around the retaining ring, the probes scan lines that are interlaced between the scanned lines of the prior carriage rotation. By interlacing scan lines, the distance between scan lines is less than the distance between the probes on the carriage. By reducing the distance between scan lines, the ability of the eddy current instrument to detect small cracks and flaws in a material is enhanced.
Some signal noise will be present in the coil current. It is not practical to mechanically eliminate all of the sources of signal noise. Accordingly, signal processing techniques are used to discriminate the current signals attributable to variations in the eddy current from noise and other undesirable signals. The principal signal processing technique employed in the present invention is to compare two coil signals that are nearly identical but for the desired eddy current signal. A split coil eddy current probe provides these two similar current signals.
Both coils have the same drive current and are drawn along adjacent parallel paths over the surface of the retaining ring. Both coils are magnetically coupled to the eddy currents that they each separately induce into the retaining ring surface. The gap between the ring surface and the probe is the same for both coils. Accordingly, the coil currents for each coil are substantially the same.
The two coils are far enough apart so that they will not pass over the same material flaws in the ring surface at the same time. Although the coils within each probe are side by side and very close together, they do not project overlapping magnetic fields onto the same portion of the ring surface. The magnetic fields generated by the each of coils and projected against the ring surface has a shape substantially the same as that of the end of the coil. The side-by-side coils project side by side magnetic fields onto the ring surface.
Since the flaws in the retaining ring material tend to be microscopic, individual flaws generally do not traverse across the side-by-side magnetic fields. When one probe coil passes over a particular feature of a material flaw, the other coil does not pass over the same flaw feature. Since material flaws affect the eddy currents that magnetically couple a coil, the current in the coil passing over the flaw is affected by the altered eddy current while the other coil current is not affected by the flaw. Accordingly, the difference between the two coil current signals is due to microflaws in the ring surface and sub-surface.
In addition, most of the noise and other signal effects in the eddy current probe can be masked from the coil signal by using impedance bridge and amplifier circuits to process the coil current signals. These circuits are contained in a conventional eddy current instrument such as an MIZ-40 model instrument manufactured by the Zetec Corp. of Issaquah, Washington. Similarly, it is acceptable to use two or more single probe channel instruments such as a model 19E, Phase II, instrument manufactured by Stanley NDJ Technologies, Inc., Kennewick, Washington, and modified for wide band width operation.
To further refine the signals from the eddy current probe, the coil signals from the bridge circuit are passed through two synchronous differential amplifier circuits to create two difference signals. One amplifier is synchronized with the drive oscillator. The in-phase signal, after the addition of a user selectable display phase angle (.phi.), can be rotated on the display screen so that it generally corresponds to liftoff variations between the two coils (to the extent that such variations exist with side-by-side coils) and other noise.
The second synchronous differential amplifier has a 90.degree. phase shift with the drive oscillator and compares the out-of-phase differences between the coil signals. The out-of-phase signal, after the addition of the same user selectable display phase angle (.phi.), is rotated on the display or plot so that it generally corresponds to variations in the eddy currents between the two coils and which are due to material flaws. Since the eddy currents are generated by and magnetically coupled to the coil current, the eddy currents are slightly behind the phase of the col 1 current. The eddy currents tend to retard the coil current because of the magnetic coupling. The currents in the two coils will be out-of-phase due to the eddy currents. Accordingly, the out-of-phase signal is more indicative of material flaws than is the in-phase signal.
Signal processors are used to digitize the eddy current signals and to present the signals in a coherent manner to the operators. The signals (in-phase and out-of-phase) from each probe are centered to account for drifting, filtered to eliminate noise, normalized and collated so as to show spatially the size and location of flaws and cracks in the surface of a retaining ring.
The processed signal data from the eddy current probe is displayed via conventional display means. Strip charts have been used to show each scan line of the probe and show where the eddy current varies with respect to the material surface. Similarly, CRT display screens can be used to present the eddy current data. The display may be adjusted to show the in-phase and out-of-phase differential signals on respective horizontal and vertical display axes to enhance the user's ability to analyze the data. In addition, a computer can be used to display the signals, and to color-code and plot the signals for a display or to print a paper copy of the data. The displays have in common the presentation of data indicative of material flaws in the ring. The data may be presented such that the location of the flaws in the material is apparent or may be presented such that the area and extent of the flaws are apparent.
It is an object of the present invention to provide an improved eddy current probe, carriage and signal processor. The carriage holds multiple probes to reduce the time needed to scan a surface. An interlaced straight lined scanning method reduces the distance between scan lines to less than the distance between probes on the carriage. A signal processing technique is used to segregate current variation due to material flaws from current variations due to noise and lift off.